The CIA thwarted an ambitious plot by al-Qaida's affiliate in Yemen to destroy a U.S.-bound airliner using a bomb with a sophisticated new design around the one-year anniversary of the killing of Osama bin Laden, The Associated Press has learned.

The plot involved an upgrade of the underwear bomb that failed to detonate aboard a jetliner over Detroit on Christmas 2009. This new bomb was also designed to be used in a passenger's underwear, but this time al-Qaida developed a more refined detonation system, U.S. officials said.

The FBI is examining the latest bomb to see whether it could have passed through airport security and brought down an airplane, officials said. They said the device did not contain metal, meaning it probably could have passed through an airport metal detector. But it was not clear whether new body scanners used in many airports would have detected it.

The would-be suicide bomber, based in Yemen, had not yet picked a target or bought his plane tickets when the CIA stepped in and seized the bomb, officials said. It's not immediately clear what happened to the alleged bomber.

The operation unfolded even as the White House and Department of Homeland Security assured the American public that they knew of no al-Qaida plots against the U.S. around the anniversary of bin Laden's death.

The AP learned about the thwarted plot last week but agreed to White House and CIA requests not to publish it immediately because the sensitive intelligence operation was still under way. Once those concerns were allayed, the AP decided to disclose the plot Monday despite requests from the Obama administration to wait for an official announcement Tuesday.

U.S. officials, who were briefed on the operation, insisted on anonymity to discuss the case.

It's not clear who built the bomb, but, because of its sophistication and its similarity to the Christmas bomb, authorities suspected it was the work of master bomb maker Ibrahim Hassan al-Asiri. Al-Asiri constructed the first underwear bomb and two others that al-Qaida built into printer cartridges and shipped to the U.S. on cargo planes in 2010.

Both of those bombs used a powerful industrial explosive. Both were nearly successful.

The operation is an intelligence victory for the United States and a reminder of al-Qaida's ambitions, despite the death of bin Laden and other senior leaders. Because of instability in the Yemeni government, the terrorist group's branch there has gained territory and strength. It has set up terrorist camps and, in some areas, even operates as a de facto government.

But along with the gains there also have been losses. The group has suffered significant setbacks as the CIA and the U.S. military focus more on Yemen. On Sunday, Fahd al-Quso, a senior al-Qaida leader, was hit by a missile as he stepped out of his vehicle along with another operative in the southern Shabwa province of Yemen.

Al-Quso, 37, was on the FBI's most wanted list, with a $5 million reward for information leading to his capture. He was indicted in the U.S. for his role in the 2000 bombing of the USS Cole in the harbor of Aden, Yemen, in which 17 American sailors were killed and 39 injured.

Al-Quso was believed to have replaced Anwar al-Awlaki as the group's head of external operations.

A plot by al-Qaeda in Yemen to detonate an upgraded version of the failed 2009 "underwear bomb" has been disrupted, US officials say.

The device was seized by intelligence operatives and is in US custody undergoing technical and forensic analysis, the FBI has said.

Reports said no target had been chosen and no plane tickets purchased by the time the alleged plot was foiled.

There is no indication on the status of the would-be bomber. As a result of close co-operation with our security and intelligence partners overseas, an improvised explosive device (IED) designed to carry out a terrorist attack has been seized abroad," the FBI said in a statement.

"Initial exploitation indicates that the device is very similar to IEDs that have been used previously by al-Qaeda in the Arabian Peninsula (AQAP) in attempted terrorist attacks, including against aircraft and for targeted assassinations," it added.

President Barack Obama was first informed of the plot in April, White House spokeswoman Caitlin Hayden said, adding that the device did not pose a threat to the public.

The BBC's Mark Mardell says that the plot, disrupted as a result of US co-operation with other unnamed intelligence agencies, appears to have been caught at a relatively early stage - after the making of the bomb but before the public was put at risk.

"What this incident makes clear is that this country has to continue to remain vigilant against those that would seek to attack this country. And we will do everything necessary to keep America safe," Defence Secretary Leon Panetta told reporters on Monday.

'Hallmarks'

It is not clear who built the device, but officials say it shares some features with the bomb sewn into the underwear of would-be suicide bomber Umar Farouk Abdulmutallab. He was arrested when his device failed to explode fully while on a plane bound for Detroit on Christmas Day 2009.

A US intelligence official said the latest device bore the "hallmarks" of the 2009 underwear bomb. It included a powerful industrial explosive and did not use metal, suggesting the bomb could have passed through airport security unnoticed, the Associated Press reports.

It is not clear whether body scanners, which operate in many US airports, would have been able to pick up the upgraded "underwear bomb".

Abdulmutallab's "underwear bomb" was not detected during security checks at airports in Lagos, Nigeria, and Amsterdam before he boarded Northwest Airlines Flight 253 for Detroit.

On the flight, the bomb did not detonate fully and passengers had to put out the fire. He has been sentenced to life in jail.

A video simulation shows what would have happened if Umar Farouk Abdulmutallab's attack had succeeded VIDEO

In a statement, the Department of Homeland Security said air security would continue to incorporate threat and vulnerability analysis, pre-screening and screening of passengers, as well as random searches at airports, air marshals and other unspecified security measures.

It added: "We have no specific, credible information regarding an active terrorist plot against the US at this time, although we continue to monitor efforts by al-Qaeda and its affiliates to carry out terrorist attacks, both in the homeland and abroad."

News of the operation emerged shortly after the US marked one year since the death of al-Qaeda leader Osama Bin Laden.

It also comes one day after Fahd al-Quso, a senior al-Qaeda leader in Yemen, was killed by a US drone strike.

The US had offered a $5m (£3.1m) reward for information leading to Quso's capture or death.

The Yemeni government has stepped up its battle against AQAP since Ali Abdullah Saleh stepped down as president in November.

However, the group and its allies still control large parts of the country.

Ray, shared security measures between the UK and the US are remarkable. Imagine being able to be on top of this type of terrorist attack. The terrorists aren't going away. Meanwhile, Obama leaves Iraq and Afghanistan vulnerable.

We need Mitt Romney in November to bring real change to our country. Obama doesn't have any interest in protecting America. As he flies around the country thumping his chest that he killed bin Laden it confirms that Obama is only interested in the terrorists if it serves him well in his campaign efforts.

This has been a sad yet dangerous four years for Americans. And yet, the liberals will still vote for Obama...one more time.

Ray, thank you for added information. It is really apparent that he doesn't take the terrorism seriously;why else would he allow the terrorists from 9/11 to be on trial in civilian court when they are military prisoners? They are making a mockery of the judicial system and he is allowing that, too.

White House counter-terrorism advisor John Brennan refused to reveal how the bomb plot was broken up

A plot by al-Qaeda in the Arabian Peninsula to attack a US-bound plane using an updated "underwear bomb" was foiled by an insider infiltrating a terror cell, US officials say.

The seized device is being examined by the FBI in Quantico, Virginia, while the source is said to have left Yemen.

The White House counter-terrorism says the bomb was never an "active threat".

Meanwhile, a senior US congressman has linked the plot to an al-Qaeda leader killed in Yemen on Sunday.

Fahd al-Quso, a senior figure in Yemen-based al-Qaeda in the Arabian Peninsula (AQAP) was killed by a drone strike.

Saudi tip

Speaking on Tuesday, White House counter-terrorism chief Brennan would not be drawn on the nature of the operation to seize the device, instead describing the aim of the FBI investigation into the device.

"Now we're trying to make sure that we take the measures that we need to prevent any other type of IED [improvised explosive device], similarly constructed, from getting through security procedures," Mr Brennan said.

Without giving specifics, the US says multiple overseas intelligence agencies were involved in the operation to seize the device.

Reports did not detail which foreign agencies the insider was working with.

However, reports have linked the device to a Saudi-born al-Qaeda bomb-maker, Ibrahim al-Asiri, previously named as a ley figure in the 2009 underwear bomb plot.

A US intelligence source told CNN the latest plan was thwarted two weeks ago following a tip from Saudi Arabia, heightening suggestions that Saudi intelligence operatives could have been involved.

Senior Yemeni officials say the government in Sanaa has no information on this particular plot, Reuters news agency reported on Tuesday.

As details of the plot emerged in the US, officials said it appeared that AQAP leaders in Yemen had instructed a suicide bomber to board any flight of his choosing to the US with the bomb under his clothes.

However, he had been stopped before reaching an airport.

Reports say no target had been chosen and no plane tickets purchased by the time the alleged plot was foiled.

Christmas Day attack

Speaking late on Monday, Republican Congressman Peter King said late on Monday that the operation was linked to the strike that killed al-Quso.

"I was told by the White House that they are connected, that they are part of the same operation," he said.

Al-Quso was a leader of AQAP based in Yemen, and the US offered a $5m (£3.1m) reward for information leading to his capture or death.

US officials told ABC News that that al-Quso was planning an attack similar to a failed 2009 attempt to blow up a passenger plane.

The alleged device seized from the Yemen cells shares some features with the bomb sewn into the underwear of would-be suicide bomber Umar Farouk Abdulmutallab during that attempt, officials said.

The Nigerian was arrested when his device failed to explode fully while on a plane bound for Detroit on Christmas Day 2009.

A US intelligence official said the latest device bore the "hallmarks" of the 2009 underwear bomb, which was built by the Saudi militant Ibrahim al-Asiri.

It seems it is an improved model, with a more effective detonation system; it has no metal parts and probably would not have been detected by most airport security magnetometers, our correspondent adds.

It is not even clear if it would have been found by the body scanners that have been installed in some US airports after that attempted attack three years ago.

In a statement, the Department of Homeland Security said air security would continue to incorporate threat and vulnerability analysis, pre-screening and screening of passengers, as well as random searches at airports, air marshals and other unspecified security measures.

'Bomb plot': Unanswered Questions

Unclear who the would-be bomber is or whether he/she is in custody

Country where alleged device was seized as yet unspecified

No information on how the alleged plot was intercepted

Exact composition of alleged device unclear, although said to contain no metal parts and designed to escape detection by magnetometers at airport security

Airport security around the world was tightened after the 2009 underwear bomb attempt

Reports from the US say the would-be suicide attacker in a foiled "underwear bomb" plot was in fact a double agent.

US officials are quoted as saying that the person dispatched by Yemen-based al-Qaeda to attack a US-bound plane had infiltrated the group.

In an apparent intelligence coup, the agent left Yemen with the device and delivered it to the CIA.

Meanwhile, the Pentagon says it is sending military trainers back to Yemen to help counter al-Qaeda militants.

US intelligence learned last month that militants with al-Qaeda in the Arabian Peninsula (AQAP) in Yemen planned to attack a plane with a more sophisticated version of a bomb hidden in a passenger's underwear, similar to one used in a failed 2009 attempt, Associated Press news agency reported.

Officials told US media that the would-be bomber had been recruited by Saudi Arabia's intelligence agency and sent to Yemen where he infiltrated the militants' cell.

The BBC's Steve Kingstone in Washington says the double-agent was reportedly given an ambitious task by Saudi intelligence - to convince AQAP that he wanted to blow up himself and a US-bound aircraft.

The agent was given the device which he then delivered to the CIA and Saudi officials.

The New York Times reports that the double-agent is now safe in Saudi Arabia.

FBI analysts are studying the device.

The upgraded underwear bomb is described by officials as a "custom-fit" device, that would have been difficult to detect even with careful security checks.

It is said to have two forms of detonator, unlike a similar bomb, from the same Yemeni source, that failed to explode on Christmas Day 2009.

The main charge was a high-grade military explosive that "undoubtedly would have brought down an aircraft", an official told the New York Times.

Experts quoted in the US media say both bombs bear the hallmarks of AQAP master bomb-maker Ibrahim Hassan Tali al-Asiri.

"The plot itself indicates that the terrorists keep trying... to devise more and more perverse and terrible ways to kill innocent people," US Secretary of State Hillary Clinton said during a visit to India.

Once again the loose lips in the Obumbler administration has given us (and our enemies) more info than we really need to know. Like our interrogation techniques we have given them confirmation of infiltration attempts. Despite The One's "successes" with OBL and drone attacks we seen to be giving away too much so The One can retain his throne.

The man who foiled a Yemen-based al-Qaeda "underwear bomb" plot is reported to have been a British man of Middle Eastern origin, according to US media.

UK officials declined to comment on the reports, but earlier this week the US said foreign agencies were involved in the operation.

The agent was sent by al-Qaeda to attack a US-bound plane, but left Yemen and gave the device to US intelligence

The US insists the bomb never posed a threat to the public.

Western passport-holders have long been seen as prime targets for militant groups seeking to plan attacks.

However, as well as reporting that the secret agent was a British citizen,NBC News in the US also says UK intelligence agencies were "heavily involved" in his recruitment.

The BBC's Paul Adams, in Washington, says it is unorthodox for intelligence agencies to disclose details about each others' operations.

If true, the revelations about the undercover agent could raise eyebrows in UK intelligence agencies, he adds.

'Custom fit'

Details of the operation have emerged over the course of this week.

US intelligence learned in April of a plot by al-Qaeda in the Arabian Peninsula (AQAP), based in Yemen.

The group planned to attack a US-bound plane with a sophisticated bomb hidden in a passenger's underwear, an updated version of a 2009 device that evaded airport security.

Officials have described the bomb as a "custom fit" device that would have been hard to detect in airport security checks.

It was said to have two forms of detonator and no metal parts, making it more sophisticated than the device that failed to explode on Christmas Day 2009.

It then emerged that the would-be bomber was an undercover agent who had been recruited to infiltrate the group.

When he was dispatched by al-Qaeda to undertake the suicide mission, the agent left Yemen with the bomb and delivered it to the CIA.

The agent is now reported to be safe in Saudi Arabia while FBI analysts in the US are studying the device.

USS Cole link killed

Experts quoted in the US media say both the 2012 and 2009 bombs bear the hallmarks of AQAP master bomb-maker Ibrahim Hassan Tali al-Asiri.

His name has also been linked to a plot in October 2010 to bomb a Chicago-bound plane using a device disguised as a printer cartridge. That plan, which AQAP claimed as its own, was also foiled with the help of the Saudi authorities.

The undercover agent also provided intelligence that led the CIA to conduct a drone strike in Yemen on Sunday that killed AQAP leader Fahd al-Quso.

Quso was wanted in connection with the bombing of the American destroyer USS Cole in Yemen 12 years ago. The US was offering a $5m (£3.1m) reward for information leading to his capture or death.

In a separate development on Tuesday the Pentagon said the US had restarted military training with security forces in Yemen, which had been put on hold because of political unrest.